JP5396092B2 - Spark plug - Google Patents

Description

  The present invention relates to a spark plug in which a ground-like electrode is provided with a needle-like ignition portion that forms a spark discharge gap with a center electrode.

  In recent years, there has been a demand for strengthening countermeasures against environmental pollution caused by exhaust gas discharged from internal combustion engines, and the improvement of ignitability contributes to the purification of exhaust gas. There is an electrode tip (discharge part) formed using a noble metal having a high height so as to protrude toward the center electrode. In the spark plug of this configuration, the ground electrode can be moved away from the spark discharge gap as compared with the conventional one, so that the flame nucleus formed in the spark discharge gap is less likely to contact the ground electrode in the initial stage of the growth process. . For this reason, since the so-called flame extinguishing action, in which the growth is hindered by contact of the flame core with the ground electrode and deprived of heat, is reduced, the ignitability of the spark plug can be improved.

  In such a form of spark plug, since a large thermal load is applied to the electrode tip, there is a possibility that cracks, peeling, and the like may occur at the joint portion between the discharge portion and the ground electrode. Therefore, in joining the discharge part (ignition part) and the ground electrode, there is one in which a pedestal part (protrusion part) is interposed as an intermediate member having a linear expansion coefficient intermediate between the two. With this pedestal portion, it is possible to reduce the occurrence of cracks, peeling, and the like by relaxing thermal stress that can occur at the joints of the discharge portion, the pedestal portion, and the ground electrode (see, for example, Patent Document 1). Further, in Patent Document 1, the electrode tip and the intermediate member are not joined by resistance welding in which an excessive pressure contact force acts during joining, heat concentration is easy and the melting depth can be increased, and internal stress after joining can be increased. This is done by laser welding where it is difficult to remain. And by this laser welding, the fusion | melting part which each component material (component) mixed is formed between an electrode tip and an intermediate member.

JP-A-11-204233

  However, the discharge part and the pedestal part expand when subjected to a heat load accompanying combustion of the engine and cause deformation in each. However, the structure, such as the formation position, size, and shape of the fusion part formed between the two parts Depending on the form, the melted part may have a structure that suppresses deformation of the discharge part and the pedestal part. In particular, when the melting portion is formed so as to connect the side surface of the discharge portion and the surface on the protruding tip side of the pedestal portion, inward in the radial direction perpendicular to the protruding direction in which the discharge portion protrudes from the ground electrode In addition, the molten part is in a form to support the discharge part. The same applies to the interface between the melted part and the pedestal part. The melted part suppresses the expansion in the radial direction (particularly outward) due to the thermal expansion of the discharge part and the pedestal part, and the respective interfaces. When the internal stress is increased, cracks and peeling may occur.

  The present invention has been made to solve the above-described problems, and is a structural form of a melting portion formed at a joint portion between a discharge portion and a pedestal portion constituting a firing portion protruding from a ground electrode. It is an object of the present invention to provide a spark plug that can suppress the occurrence of cracks and peeling.

The spark plug according to the present invention has a center electrode, an axial hole extending along the axial direction, an insulator that holds the center electrode inside the axial hole, and surrounds and holds the insulator in the circumferential direction. A metal shell, one end of which is joined to the metal shell, a ground electrode that is bent so that one side surface of the other end faces the tip of the center electrode, and the other end of the ground electrode On the one side surface, a spark plug provided at a position facing the tip portion of the center electrode and projecting from the one side surface toward the center electrode, the ignition portion includes the following: It has characteristics. First, the ignition portion is joined by laser welding to a pedestal portion projecting from the one side surface toward the center electrode, and a projecting tip of the pedestal portion, and a spark is formed between itself and the tip portion of the center electrode. It has a discharge part that forms a discharge gap, and a fusion part that is interposed between the pedestal part and the discharge part, and is formed by melting the constituent materials of each other by the laser welding. And when the said ignition part sees arbitrary cross sections of the said ignition part including the central axis of the said ignition part along the direction which protrudes from the said one side surface of the said ground electrode, the said fusion | melting part is a side surface of the said ignition part The melting portion is connected to the side surface of the pedestal portion and the side surface of the discharge portion when the contour shape of the arbitrary cross section of the ignition portion is viewed. It has a shape. Furthermore, in the arbitrary cross section of the ignition part, the position of the boundary between the pedestal part and the melting part on one of the side surfaces is X1, and the position of the boundary between the discharge part and the melting part is X2. and then while, when the straight line distance between the position X1 and X2 of the boundary of the arbitrary cross-section viewing the first section having a maximum, and the outer diameter S of the discharge portion in a radial direction perpendicular to the central axis The length T of the melted portion extending inward in the radial direction with respect to the position X2 of the boundary between the discharge portion and the melted portion satisfies T / S ≧ 0.5, and the boundary position X1 and a virtual line passing through the X2, external angle theta of the central axis and forms angle, meets the 135 ° ≦ θ ≦ 175 °, and the side surface of the pedestal portion, the one side surface of the ground electrode to which the base portion is provided Is a cross-sectional shape including the central axis of the ignition portion It is connected by a first connecting portion forming a recessed curved shape on the inside.

  In the spark plug according to the present invention, the melting portion is formed over the entire circumference in the circumferential direction of the ignition portion. That is, in the radial direction of the ignition portion, the discharge portion or the pedestal portion is configured to be held inward in the radial direction by the melting portion at a portion where the discharge portion or the pedestal portion and the melting portion are arranged in layers. Therefore, when the discharge part and the pedestal part are stretched (deformed) in the radial direction by receiving heat, the discharge part and the pedestal part are subjected to a drag force by expanding the annular melt part continuously outward in the circumferential direction of the ignition part, Stretching is suppressed. Here, when the outline shape of the cross section of the ignition part is seen, the melting part is connected to the side surface of the pedestal part and the side surface of the discharge part. For this reason, compared with the case where a fusion part connects the side surface of a discharge part, and the plane (for example, one side surface of a ground electrode, or the front end surface of a base part) which spreads along the radial direction of a firing part, the diameter of a discharge part. It is possible to reduce the suppression by the melted portion with respect to the outward extension in the direction.

  Further, according to the spark plug according to the present invention, in the first cross section of the ignition portion, the outer angle θ of the angle formed by the imaginary line passing through the position X1 and the position X2 and the central axis of the ignition portion is 135 ° ≦ θ It satisfies that ≦ 175 °. When the outer angle θ is less than 180 °, the taper shape formed by the melted portion becomes a form spreading from the position X2 toward the position X1, and at the position X2, the melted part presses the discharge part radially inward. The smaller the outer angle θ and the greater the extent of the taper, the more easily the melted part itself can withstand the radially outward pressing force. For this reason, when the discharge part receives heat and deformation due to thermal expansion occurs, the outward deformation in the radial direction of the discharge part is easily suppressed by the melting part, and specifically, when the outer angle θ is smaller than 135 °, Internal stress increases at the interface between the discharge part and the melted part, which may cause cracks and peeling. On the other hand, when the pedestal portion having a larger linear expansion coefficient than the discharge portion is deformed due to thermal expansion, the pedestal portion is deformed larger than the discharge portion. It is easy to receive suppression from. Even if the outer angle θ is less than 180 ° and the taper shape formed by the melting portion is widened from the position X2 toward the position X1, the pedestal portion is easily affected by suppression of the deformation of the melting portion. Specifically, when the outer angle θ is greater than 175 °, internal stress increases at the interface between the pedestal portion and the melted portion, which may cause cracks and peeling.

  By the way, although the ignition part is arranged at a position facing the tip part of the center electrode, the term “opposite” in the present invention strictly refers to a state in which the opposing surfaces of the tip part and the ignition part are arranged in parallel. However, it does not mean a configuration in which the center electrode and the ignition part are precisely aligned. That is, it is sufficient if a spark discharge gap is formed between the tip portion of the center electrode and the ignition portion when predetermined power is supplied to the spark plug of the present invention.

Furthermore, according to the spark plug according to the present invention, when the ratio of the formation depth of the melted part to the outer diameter S of the discharge part (melted part forming ratio) is determined as T / S in an arbitrary cross section of the ignition part, It satisfies that T / S ≧ 0.5. It is preferable to interpose a molten part having a linear expansion coefficient between the discharge part and the pedestal part between them in order to relieve the thermal stress generated between them. The larger the length (formation depth) T at which the melted portion extends inward, the greater the size of the melted portion between the discharge portion and the pedestal portion, so that the thermal stress generated between them is alleviated. It becomes easy. Specifically, if the melted part is formed so that T / S is 0.5 or more, the occurrence of cracks and peeling can be effectively suppressed.
Further, in the spark plug according to the present invention, the side surface of the pedestal portion and the one side surface of the ground electrode provided with the pedestal portion are recessed inward in the shape of a cross section including the central axis of the ignition portion. It is connected by a first connecting part having a curved shape. Since the ignition part is provided in a form protruding from one side surface of the ground electrode, if the thickness is increased by providing the first connecting part at the base part, for example, even when subjected to vibration or the like associated with driving the engine, A structure that can withstand the load caused by the vibration can be obtained.
The spark plug according to the present invention has a center electrode, an axial hole extending along the axial direction, an insulator that holds the center electrode inside the axial hole, and surrounds and holds the insulator in the circumferential direction. A metal shell, one end of which is joined to the metal shell, a ground electrode that is bent so that one side surface of the other end faces the tip of the center electrode, and the other end of the ground electrode On the one side surface, a spark plug provided at a position facing the tip portion of the center electrode and projecting from the one side surface toward the center electrode, the ignition portion includes the following: It has characteristics. First, the ignition portion is joined by laser welding to a pedestal portion projecting from the one side surface toward the center electrode, and a projecting tip of the pedestal portion, and a spark is formed between itself and the tip portion of the center electrode. It has a discharge part that forms a discharge gap, and a fusion part that is interposed between the pedestal part and the discharge part, and is formed by melting the constituent materials of each other by the laser welding. And when the said ignition part sees arbitrary cross sections of the said ignition part including the central axis of the said ignition part along the direction which protrudes from the said one side surface of the said ground electrode, the said fusion | melting part is a side surface of the said ignition part The melting portion is connected to the side surface of the pedestal portion and the side surface of the discharge portion when the contour shape of the arbitrary cross section of the ignition portion is viewed. It has a shape. Furthermore, in the arbitrary cross section of the ignition part, the position of the boundary between the pedestal part and the melting part on one of the side surfaces is X1, and the position of the boundary between the discharge part and the melting part is X2. However, when the first cross section where the linear distance between the boundary positions X1 and X2 is the maximum among the arbitrary cross sections is viewed, the outer diameter S of the discharge portion in the radial direction perpendicular to the central axis The length T of the melted portion extending inward in the radial direction with respect to the position X2 of the boundary between the discharge portion and the melted portion satisfies T / S ≧ 0.5, and the boundary position X1 and An outer angle θ formed by an imaginary line passing through X2 and the central axis satisfies 135 ° ≦ θ ≦ 175 °, and the pedestal portion has its outer diameter enlarged on the one side surface of the ground electrode. It has a collar part that is formed into a diameter, and the collar part of the pedestal part And the side facing the protruding tip side and the side surface of the pedestal portion at the protruding tip rather than the flange portion are formed in a curved shape in which the shape of the cross section including the central axis of the ignition portion is recessed inward. It is connected by a connecting part.
In the spark plug according to the present invention, the melting portion is formed over the entire circumference in the circumferential direction of the ignition portion. That is, in the radial direction of the ignition portion, the discharge portion or the pedestal portion is configured to be held inward in the radial direction by the melting portion at a portion where the discharge portion or the pedestal portion and the melting portion are arranged in layers. Therefore, when the discharge part and the pedestal part are stretched (deformed) in the radial direction by receiving heat, the discharge part and the pedestal part are subjected to a drag force by expanding the annular melt part continuously outward in the circumferential direction of the ignition part, Stretching is suppressed. Here, when the outline shape of the cross section of the ignition part is seen, the melting part is connected to the side surface of the pedestal part and the side surface of the discharge part. For this reason, compared with the case where a fusion part connects the side surface of a discharge part, and the plane (for example, one side surface of a ground electrode, or the front end surface of a base part) which spreads along the radial direction of a firing part, the diameter of a discharge part. It is possible to reduce the suppression by the melted portion with respect to the outward extension in the direction.
Further, according to the spark plug according to the present invention, in the first cross section of the ignition portion, the outer angle θ of the angle formed by the imaginary line passing through the position X1 and the position X2 and the central axis of the ignition portion is 135 ° ≦ θ It satisfies that ≦ 175 °. When the outer angle θ is less than 180 °, the taper shape formed by the melted portion becomes a form spreading from the position X2 toward the position X1, and at the position X2, the melted part presses the discharge part radially inward. The smaller the outer angle θ and the greater the extent of the taper, the more easily the melted part itself can withstand the radially outward pressing force. For this reason, when the discharge part receives heat and deformation due to thermal expansion occurs, the outward deformation in the radial direction of the discharge part is easily suppressed by the melting part, and specifically, when the outer angle θ is smaller than 135 °, Internal stress increases at the interface between the discharge part and the melted part, which may cause cracks and peeling. On the other hand, when the pedestal portion having a larger linear expansion coefficient than the discharge portion is deformed due to thermal expansion, the pedestal portion is deformed larger than the discharge portion. It is easy to receive suppression from. Even if the outer angle θ is less than 180 ° and the taper shape formed by the melting portion is widened from the position X2 toward the position X1, the pedestal portion is easily affected by suppression of the deformation of the melting portion. Specifically, when the outer angle θ is greater than 175 °, internal stress increases at the interface between the pedestal portion and the melted portion, which may cause cracks and peeling.
By the way, although the ignition part is arranged at a position facing the tip part of the center electrode, the term “opposite” in the present invention strictly refers to a state in which the opposing surfaces of the tip part and the ignition part are arranged in parallel. However, it does not mean a configuration in which the center electrode and the ignition part are precisely aligned. That is, it is sufficient if a spark discharge gap is formed between the tip portion of the center electrode and the ignition portion when predetermined power is supplied to the spark plug of the present invention.
Furthermore, according to the spark plug according to the present invention, when the ratio of the formation depth of the melted part to the outer diameter S of the discharge part (melted part forming ratio) is determined as T / S in an arbitrary cross section of the ignition part, It satisfies that T / S ≧ 0.5. It is preferable to interpose a molten part having a linear expansion coefficient between the discharge part and the pedestal part between them in order to relieve the thermal stress generated between them. The larger the length (formation depth) T at which the melted portion extends inward, the greater the size of the melted portion between the discharge portion and the pedestal portion, so that the thermal stress generated between them is alleviated. It becomes easy. Specifically, if the melted part is formed so that T / S is 0.5 or more, the occurrence of cracks and peeling can be effectively suppressed.
Furthermore, the pedestal portion has a flange portion whose outer diameter is enlarged on the one side surface side of the ground electrode. In this case, the surface of the pedestal portion facing the protruding tip side in the flange portion and the side surface of the pedestal portion at the protruding tip end than the flange portion have a cross-sectional shape including the central axis of the ignition portion. It is connected by a second connecting portion having a curved shape recessed inward. When the collar portion is provided on the pedestal portion, the stability of joining the pedestal portion to one side surface of the ground electrode can be enhanced. And if a 2nd connection part is provided between the collar part and the side surface of the main body of a base part, and a thickness is increased, the ignition part is enough for the load of vibration etc. concerning its root part like the above. An endurable structure can be obtained.

  Moreover, in the spark plug according to the present invention, among the plurality of cross sections observed at different circumferential positions around the central axis, the cross section of more than half of the entire circumference is the arbitrary cross section of the ignition portion. The outer diameter S and the length T satisfy T / S ≧ 0.5, and the outer angle θ preferably satisfies 135 ° ≦ θ ≦ 175 °. The above-mentioned provisions of 135 ° ≦ θ ≦ 175 ° and T / S ≧ 0.5 are observed not only in the first cross section but also in an arbitrary cross section of the ignition part at different circumferential positions around the central axis. Of the plurality of cross sections, it is preferable that the cross section is filled in half or more of the entire circumference. When forming the melted part, for example, when spot welding is intermittently performed around the ignition part, the formed melted part is unlikely to have a uniform shape over the entire circumference of the ignition part. The larger the gap is, the more greatly the shape and size of the melted portion varies depending on the cross section. In such a case, among the plurality of cross-sections observed at different circumferential positions around the central axis in any cross-section of the ignition portion, the cross-sections that do not satisfy the above-mentioned definition increase. Of the above-mentioned arbitrary cross sections of the ignition part, if the above definition is satisfied in at least half of the entire circumference, even if there is a place where the internal stress partially increases at each interface of the discharge part, the pedestal part and the melting part The internal stress can be easily dispersed, and the occurrence of cracks and peeling can be effectively suppressed.

In the spark plug according to the present invention, a difference between a linear expansion coefficient of a material constituting the discharge part of the ignition part and a linear expansion coefficient of a material constituting the pedestal part is 8.1 × 10 ⁻⁶ [ 1 / K] or less. In this way, when the discharge part and the pedestal part are stretched (deformed) in the radial direction during heat reception, the difference in internal stress generated at the interface between the discharge part and the melt part is limited, and the deviation of the internal stress is suppressed. Therefore, the occurrence of cracks and peeling can be more effectively suppressed.

Further, in the spark plug according to the present invention, the front Symbol pedestal, on the one side surface of the ground electrode may have a flange portion outer diameter of its own, which are expanded. In this case, the surface of the pedestal portion facing the protruding tip side in the flange portion and the side surface of the pedestal portion at the protruding tip end than the flange portion have a cross-sectional shape including the central axis of the ignition portion. You may be connected by the 2nd connection part which makes the curved shape dented inside . Case in which the flange portion to the pedestal portion, with respect to one side surface of the ground electrode, it is possible to increase the bonding stability of the pedestal. And if a 2nd connection part is provided between the collar part and the side surface of the main body of a base part, and a thickness is increased, the ignition part is enough for the load of vibration etc. concerning its root part like the above. A durable structure can be obtained and is desirable.

  In the spark plug according to the present invention, the discharge portion of the ignition portion may be formed using any single noble metal of Pt, Ir, Rh, or Ru, or the noble metal Of these, a noble metal alloy containing at least one or more noble metals may be used. It is desirable to form a discharge portion that forms a spark discharge gap with the center electrode using a noble metal or a noble metal alloy in order to obtain oxidation resistance and spark consumption resistance.

1 is a partial cross-sectional view of a spark plug 100. FIG. It is the fragmentary sectional view which expanded the spark discharge gap GAP vicinity. It is the figure which looked at the 1st cross section of the ignition part 80. FIG. It is a figure which shows the ignition part 180 as a modification. It is a figure which shows the ignition part 280 as a modification. It is sectional drawing of the ignition part 380 shown as an example for demonstrating the method to obtain | require an oxide scale.

  Hereinafter, an embodiment of a spark plug embodying the present invention will be described with reference to the drawings. First, the structure of the spark plug 100 as an example will be described with reference to FIGS. FIG. 1 is a partial cross-sectional view of a spark plug 100. FIG. 2 is an enlarged partial sectional view of the vicinity of the spark discharge gap GAP. 1 and 2, the axis O direction of the spark plug 100 is the vertical direction in the drawings, the lower side is the front end side of the spark plug 100, and the upper side is the rear end side.

  As shown in FIG. 1, the spark plug 100 generally includes an insulator 10 that holds the center electrode 20 on the front end side in its own shaft hole 12 and holds the terminal fitting 40 on the rear end side. Is surrounded and held by the metal shell 50. A ground electrode 30 is joined to the metal shell 50, and the other end (tip 31) side is bent so as to face the tip 22 of the center electrode 20.

  First, the insulator 10 of the spark plug 100 will be described. As is well known, the insulator 10 is formed by firing alumina or the like, and has a cylindrical shape in which an axial hole 12 extending in the direction of the axis O is formed at the axial center. A flange portion 19 having the largest outer diameter is formed substantially at the center in the direction of the axis O, and a rear end body portion 18 is formed on the rear end side (upper side in FIG. 1). A front end side body portion 17 having an outer diameter smaller than that of the rear end side body portion 18 is formed on the front end side (lower side in FIG. 1) from the flange portion 19, and further, the front end side is closer to the front end side than the front end side body portion 17. A long leg portion 13 having an outer diameter smaller than that of the side body portion 17 is formed. The long leg portion 13 is reduced in diameter toward the distal end side, and when the spark plug 100 is attached to the engine head (not shown) of the internal combustion engine, it is exposed to the combustion chamber. Further, a step portion 15 is formed in a step shape between the leg long portion 13 and the distal end side trunk portion 17.

  Next, the center electrode 20 will be described. The center electrode 20 is mainly composed of copper or copper, which is superior in thermal conductivity to the base material 24 inside the base material 24 formed of an alloy containing Ni as a main component, such as Inconel (trade name) 600 or 601. This is a rod-like electrode having a structure in which a core material 25 made of an alloy is embedded. The center electrode 20 is held on the distal end side in the shaft hole 12 of the insulator 10, and the distal end portion 22 of the center electrode 20 protrudes further toward the distal end side than the distal end of the insulator 10. The distal end portion 22 of the center electrode 20 is formed so that the diameter thereof becomes smaller toward the distal end side, and an electrode tip 90 made of a noble metal is provided on the distal end surface of the distal end portion 22 in order to improve spark wear resistance. It is joined.

  The center electrode 20 extends toward the rear end side in the shaft hole 12 of the insulator 10, and passes through the conductive seal body 4 and the ceramic resistor 3 extending along the axis O direction. It is electrically connected to the terminal fitting 40 (upper in FIG. 1). A high voltage cable (not shown) is connected to the terminal fitting 40 via a plug cap (not shown) so that a high voltage is applied.

  Next, the metal shell 50 will be described. The metal shell 50 is a cylindrical metal fitting for fixing the spark plug 100 to the engine head (not shown) of the internal combustion engine. The metal shell 50 holds the insulator 10 inside itself so as to surround a portion from the rear end side body portion 18 of the insulator 10 to the leg long portion 13. The metal shell 50 is formed of a low carbon steel material, and a tool engaging portion 51 to which a spark plug wrench (not shown) is fitted, and a mounting portion 52 in which a screw thread to be screwed into a mounting hole (not shown) of the engine head is formed. And.

  Further, a hook-shaped seal portion 54 is formed between the tool engaging portion 51 and the attachment portion 52 of the metal shell 50. An annular gasket 5 formed by bending a plate is fitted into the screw neck portion between the seal portion 54 and the attachment portion 52. When the spark plug 100 is mounted in the engine head mounting hole (not shown), the gasket 5 is crushed and deformed between the seating surface of the seal portion 54 and the opening periphery of the mounting hole, and seals between the two. By stopping, airtight leakage in the engine through the mounting hole is prevented.

  Next, a thin caulking portion 53 is provided on the rear end side of the metal fitting 50 from the tool engaging portion 51, and between the seal portion 54 and the tool engaging portion 51, similarly to the caulking portion 53. A thin buckling portion 58 is provided. Between the inner peripheral surface of the metal shell 50 from the tool engagement portion 51 to the crimping portion 53 and the outer peripheral surface of the rear end side body portion 18 of the insulator 10, annular ring members 6 and 7 are interposed. Furthermore, talc (talc) 9 powder is filled between the ring members 6 and 7. By crimping the crimping portion 53 so as to be bent inward, the insulator 10 is pressed toward the front end side in the metal shell 50 via the ring members 6, 7 and the talc 9. Thereby, the step portion 15 of the insulator 10 is supported by the step portion 56 formed at the position of the mounting portion 52 on the inner periphery of the metal shell 50 via the annular plate packing 8, so that the metal shell 50 and the insulator 50 are supported. 10 and unity. At this time, the airtightness between the metal shell 50 and the insulator 10 is maintained by the plate packing 8, and the outflow of combustion gas is prevented. Further, the buckling portion 58 is configured to bend outwardly and deform with the addition of a compressive force during caulking. The compression length in the direction of the axis O of the talc 9 is increased so that the inside of the metal shell 50 is increased. Increases airtightness.

  Next, the ground electrode 30 will be described. The ground electrode 30 is a bar-shaped electrode having a rectangular cross section. One end (base end 32) is joined to the front end surface 57 of the metal shell 50 and extends along the direction of the axis O while the other end (tip 31). ) Is bent so that one side surface (inner surface 33) thereof faces the front end portion 22 of the center electrode 20. Similarly to the center electrode 20, the ground electrode 30 is made of an alloy containing Ni as a main component, such as Inconel (trade name) 600 or 601.

  The tip portion 31 of the ground electrode 30 is provided with a firing portion 80 that protrudes from the inner surface 33 toward the tip portion 22 of the center electrode 20. The ignition part 80 is provided at a position facing the tip part 22 of the center electrode 20 (more specifically, the electrode tip 90 joined to the tip part 22), and a spark discharge gap GAP is formed therebetween. . It should be noted that the opposing relationship between the ignition portion 80 and the tip portion 22 of the center electrode 20 is sufficient if a spark discharge gap GAP is formed between them, and the opposing surfaces (facing surfaces) of the ignition portion 80 and the electrode tip 90 are not necessarily limited. They do not have to have a strict correspondence. Therefore, the axis O of the spark plug 100 and the central axis P (see FIG. 2) of the ignition part 80 do not have to exactly match. Here, the central axis P of the ignition part 80 is the center of its cross section orthogonal to the protruding direction of the ignition part 80 (that is, the direction in which the ignition part 80 protrudes from the inner surface 33 of the ground electrode 30 toward the center electrode 20). A straight line that passes through and parallel to the protruding direction or an approximate straight line.

  As shown in FIG. 2, the ignition part 80 includes a pedestal part 84 formed on the inner surface 33 of the ground electrode 30 and a discharge part 81 joined to the pedestal part 84. The pedestal portion 84 is formed in a columnar shape by projecting a part of the inner surface 33 toward the distal end portion 22 at a position facing the distal end portion 22 of the center electrode 20 on the inner surface 33 of the ground electrode 30. A connecting portion 89 having a cross-sectional shape recessed inward is provided at a position where the side surface 85 of the pedestal portion 84 and the inner surface 33 are combined, and the side surface 85 and the inner surface 33 are connected via the connecting portion 89. .

  The discharge part 81 also has a column shape, and is joined to the pedestal part 84 integrally by laser welding in a state of being disposed at the protruding tip 86 of the pedestal part 84. The discharge part 81 is formed using a Pt alloy, and is excellent in oxidation resistance and spark wear resistance. In addition, as a material of the discharge part 81, you may use not only Pt alloy but the single noble metal in any one of Pt, Ir, Rh, or Ru, or at least any one of these noble metals. A noble metal alloy containing two or more noble metals may be used. A fusion part 83 in which the constituent materials (components) of both are melted and mixed with each other is formed at the joint portion of the discharge part 81 and the pedestal part 84.

  In the spark plug 100 of the present embodiment configured as described above, the discharge part 81 and the pedestal part 84 constituting the ignition part 80 are joined by laser welding as described above. Specifically, the ignition part 80 is formed as follows. The ground electrode 30 is subjected to, for example, pressing or cutting, thereby forming a pedestal portion 84 that protrudes from the inner surface 33. In addition, a columnar discharge portion 81 is formed using a noble metal or a noble metal alloy, and is stacked on the protruding tip 86 of the pedestal portion 84 with the axial direction aligned. The outer diameter of the pedestal portion 84 is formed to be slightly larger than the outer diameter of the discharge portion 81. When the discharge portion 81 is disposed on the pedestal portion 84 in a state before welding, the protruding tip 86 of the pedestal portion 84 is A portion (edge portion) protrudes outward from the discharge portion 81. In this state, the side surface 82 of the discharge portion 81 and the side surface 85 of the pedestal portion 84 (that is, the side surface 87 of the ignition portion 80 after formation) are directed to the central axis P so as to aim at the mating surface of the discharge portion 81 and the pedestal portion 84. Irradiate with laser light. Thereby, between the discharge part 81 and the base part 84, the fusion | melting part 83 by which both component materials melt | dissolved and mixed was formed. At this time, the edge portion of the protruding tip 86 protruding from the discharge portion 81 is melted, and the side surface 82 of the discharge portion 81 and the side surface 85 of the pedestal portion 84 are connected by the exposed surface 88 of the melting portion 83. Laser welding is performed by making a round around the ignition part 80 in the circumferential direction of the central axis P, and the discharge part 81 and the pedestal part 84 are connected by the melting part 83. The laser beam irradiation at this time may be performed continuously or intermittently. However, when intermittently performed, the discharge unit 81 and the pedestal unit 84 when viewed from the outer peripheral side of the ignition unit 80, It is desirable that the irradiation positions of the laser beams be overlapped next to each other so that the position of the mating surface is the melting portion 83.

  In the present embodiment, the melting part 83 formed in this way is defined as follows when viewed in an arbitrary cross section including the central axis P of the ignition part 80. First, the melting part 83 is formed between the discharge part 81 and the pedestal part 84 in a form from the side surfaces 87 on both sides of the ignition part 80 toward the central axis P. Furthermore, when the outline shape of the ignition part 80 (that is, the cross-sectional shape of the exposed surface 88 of the ignition part 80) is seen in the cross section, the melting part 83 is connected to the side surface 82 of the discharge part 81 and the side surface 85 of the pedestal part 84. Form. Therefore, the exposed surface 88 of the melting part 83 is not connected to the inner surface 33 of the ground electrode 30.

  In the contour shape of the arbitrary cross section of the ignition part 80, the position of the boundary between the pedestal part 84 and the melting part 83 (the side surface 85 and the exposed surface 88 on the cross section on one side of the ignition part 80). Let X1 be the position of the boundary. Similarly, the position of the boundary between the discharge part 81 and the melting part 83 (the position of the boundary between the side surface 82 and the exposed surface 88 on the cross section) is X2. Next, the position X1 and the position X2 are connected with a straight line, and the cross section where the linear distance between the position X1 and the position X2 is the largest among the plurality of cross sections assumed as the arbitrary cross section is selected, and the ignition unit 80 One section. This first cross section is shown in FIG. In the first cross section, assuming a virtual line Q passing through the position X1 and the position X2, the virtual line Q and the central axis P are at a point C where the virtual line Q intersects the central axis P of the ignition unit 80. The external angle θ of the formed angle is obtained. At this time, in this embodiment, it is specified that 135 ° ≦ θ ≦ 175 ° is satisfied.

  The discharge part 81 made of Pt alloy has a smaller linear expansion coefficient than the ground electrode 30 made of Ni alloy and the pedestal part 84, and the melting part 83 in which both constituent materials are mixed has a linear expansion coefficient between them. Takes a value. When the ignition part 80 receives heat by driving the engine, the discharge part 81 and the pedestal part 84 including the melting part 83 are deformed by heat and are expanded. Regarding the central axis P direction, the discharge portion 81, the melting portion 83, and the pedestal portion 84 are arranged in layers, and the discharge portion 81 faces the spark discharge gap GAP. Even if the discharge portion 81, the melting portion 83, and the pedestal portion 84 are stretched (deformed), it is difficult to suppress the stretch. On the other hand, the melting portion 83 is formed inward in the radial direction while circling the side surface 87 of the ignition portion 80, so that the discharge portion 81, the pedestal portion 84, and the melting portion 83 are in the radial direction of the central axis P. The discharge part 81 and the pedestal part 84 are configured to be held inward in the radial direction by the melting part 83 at the portion having the layered arrangement. For this reason, when the discharge part 81 and the pedestal part 84 extend (deform) in the radial direction, the extension is suppressed by the melting part 83.

  Focusing on the direction connecting the position X1 and the position X2 (the direction in which the imaginary line Q extends) with respect to the cross-sectional shape of the exposed surface 88 of the melted part 83, at the position X2, the smaller the outer angle θ is, The radially inward component increases. If the melting part 83 is tapered, the melting part 83 is configured to hold the discharge part 81 having a smaller diameter than the base part 84 inward in the radial direction. And, as the degree of taper spread increases, the melted part 83 itself has a structure that can easily withstand the outward pressing force in the radial direction. For this reason, when the discharge part 81 receives heat and the deformation | transformation by thermal expansion arises, the deformation | transformation to the radial direction outward of the discharge part 81 becomes easy to be suppressed by the fusion | melting part 83 as mentioned above. For this reason, the internal stress increases at the interface between the discharge part 81 and the melting part 83, and according to Example 1 described later, if the outer angle θ is smaller than 135 °, there is a risk of causing cracks or peeling.

  On the other hand, the pedestal portion 84 has a larger linear expansion coefficient than the discharge portion 81, and when the deformation due to thermal expansion occurs, the pedestal portion 84 deforms larger than the discharge portion 81. Focusing on the direction connecting the position X1 and the position X2 (the direction in which the imaginary line Q extends) with respect to the cross-sectional shape of the exposed surface 88 of the melting portion 83, the larger the outer angle θ at the position X1, The radially outward component is reduced. That is, at the position X1, the pedestal portion 84 is more likely to be suppressed from the melting portion 83 with respect to its own deformation as the outer angle θ increases. Since the pedestal portion 84 is more deformed by thermal expansion than the discharge portion 81, the pedestal portion 84 is easily affected by the suppression of the deformation of the melting portion 83 even when the outer angle θ is less than 180 °. For this reason, according to Example 1 to be described later, when the outer angle θ is larger than 175 °, internal stress increases at the interface between the pedestal portion 84 and the melting portion 83, which may cause cracks and peeling.

  Next, in the above-mentioned arbitrary cross section of the ignition part 80 (for the sake of convenience, description will be made using the first cross section of FIG. 3), the outer diameter of the discharge part 81 in the radial direction with respect to the central axis P of the ignition part 80 is S. . The length (formation depth) at which the melted portion 83 extends radially inward with reference to the position X2 (the position of the boundary between the side surface 82 of the discharge portion 81 and the exposed surface 88 of the melted portion 83 on the cross section). T. As described above, the melting portion 83 is formed from the side surface 87 of the ignition portion 80 toward the central axis P. If the formation depth does not reach the central axis P, as shown in FIG. In the cross section of the ignition part 80, the left and right sides of the central axis P are divided into two. Therefore, in the cross section of the ignition part 80, the length T in which the melting part 83 extends radially inward is defined as the length T1 that extends radially inward on the left side of the central axis P and the radial direction on the right side of the central axis P. It is defined as the total length with the length T2 extending inward. When the ratio of the formation depth of the melted part 83 to the outer diameter S of the discharge part 81 (melted part formation ratio) is obtained as T / S, in this embodiment, T / S ≧ 0.5 is satisfied. Is stipulated.

  It is preferable to interpose a melting part 83 having a linear expansion coefficient between the discharge part 81 and the pedestal part 84 between them in terms of mitigating thermal stress generated between them. Since the length T in which the melting portion 83 extends inward from the position X2 in the radial direction of the ignition portion 80 is larger, the size of the melting portion 83 interposed between the discharge portion 81 and the pedestal portion 84 is larger. The thermal stress generated between the two can be easily relaxed, and the occurrence of cracks and peeling can be effectively suppressed. According to Example 2 to be described later, the smaller the T / S, the ratio of the size of cracks generated at the interfaces of the discharge part 81, the pedestal part 84, and the melting part 83 on the cross section of the ignition part 80 (oxidation). There was a tendency for the scale to increase. It was found that the oxide scale can be suppressed to less than 50% if the melted portion 83 is formed so that T / S is 0.5 or more.

  Further, the above-mentioned regulations, that is, 135 ° ≦ θ ≦ 175 ° and T / S ≧ 0.5 are not only the first cross section but also arbitrary cross sections of the ignition portion 80 and different circumferential directions around the central axis P Of the plurality of cross-sections observed at the position, it is preferable that the cross-sections are half or more of the entire circumference. When forming the melting part 83, for example, when spot welding is intermittently performed around the ignition part 80, the formed melting part 83 is unlikely to have a uniform shape over the entire circumference of the ignition part 80. The larger the interval between the laser beam irradiations, the more greatly the shape and size of the melted portion 83 differ depending on the cross section. In such a case, among the plurality of cross-sections that are arbitrary cross-sections of the ignition portion 80 and are observed at different circumferential positions around the central axis P, the cross-sections that do not satisfy the above-mentioned regulations increase. If the above-mentioned definition is satisfied in at least half of the entire cross section of the ignition part, the internal stress is partially increased at each interface of the discharge part 81, the pedestal part 84, and the melting part 83. Even if it exists, it becomes easy to disperse | distribute the internal stress, and there exists an effect in suppression of generation | occurrence | production of a crack, peeling, etc.

In addition, according to the result of Example 1 described later, the difference between the linear expansion coefficient of the material constituting the discharge part 81 and the linear expansion coefficient of the material constituting the pedestal part 84 is 8.1 × 10 ⁻⁶ [1 / K] Each constituent material may be selected so that the following is satisfied. In this way, when the discharge portion 81 and the pedestal portion 84 expand (deform) in the radial direction during heat reception, the difference in internal stress generated at the interface between each of the discharge portion 81 and the pedestal portion 84 is limited. Therefore, the occurrence of cracks and peeling can be more effectively suppressed.

  Further, in the present embodiment, as described above, the side surface 85 of the pedestal portion 84 and the inner surface 33 of the ground electrode 30 are connected by the connecting portion 89. Since the ignition part 80 is provided so as to protrude from the inner surface 33 of the ground electrode 30, for example, when receiving vibration or the like accompanying driving of the engine, a load due to the vibration is likely to be applied to the root part of the ignition part 80. Here, if the melting part 83 is formed in a form in which the side surface 82 of the discharge part 81 and the inner surface 33 of the ground electrode 30 are connected, the thickness of the root part of the ignition part 80 increases, and the melting part 83 causes the ignition part 80 to move. Since it becomes a form to support, the ignition part 80 can obtain the structure which can fully endure the load concerning a root part. However, in the present embodiment, in order to reduce the influence of internal stress on the interface between the melting portion 83 and the discharge portion 81 and the pedestal portion 84, the exposed surface 88 of the melting portion 83 is the side surface 82 and the pedestal of the discharge portion 81. It is configured to be connected to the side surface 85 of the portion 84. For this reason, in order to obtain a structure in which the ignition part 80 can withstand the load applied to its root part, the connecting part is provided between the side surface 85 of the base part 84 and the inner surface 33 of the ground electrode 30 as described above. 89 may be included.

  Needless to say, the present invention can be modified in various ways. For example, although the discharge part 81 and the pedestal part 84 are joined by laser welding, electron beam welding may be applied. Further, the laser welding is not limited to being performed by irradiating laser light from a direction orthogonal to the central axis P aiming at a mating surface between the discharge portion 81 and the pedestal portion 84. For example, the melting portion 83 may be formed by irradiating laser light from the oblique direction with respect to the central axis P toward the mating surface between the discharge portion 81 and the pedestal portion 84.

  Further, like the ignition part 180 shown in FIG. 4, the formation depth of the melting part 183 formed between the discharge part 81 and the pedestal part 84 reaches the central axis P, and the central axis P in the cross section of the ignition part 180 Alternatively, the melting part 183 may be formed in such a form that the part on one side and the part on the other side are continuous.

  Further, like the ignition part 280 shown in FIG. 5, the pedestal part 284 and the ground electrode 230 are formed separately, and the pedestal part 284 and the ground electrode 230 are joined together by, for example, resistance welding, The discharge part 281 may be joined by forming the fusion part 283 by laser welding as in the present embodiment. In the joining of the discharge part 281 and the pedestal part 284, it is preferable that the above-mentioned regulations are satisfied. Further, the pedestal portion 284 may have a flange portion 274 whose diameter is increased at the end on the ground electrode 230 side. By joining the flange portion 274 to the inner surface 233 of the ground electrode 230, a large joining area can be secured and more stable joining properties can be obtained. Further, if the same connecting portion 289 is provided between the side surface 275 of the flange portion 274 and the inner surface 233 of the ground electrode 230, the ignition portion 280 can withstand a load (vibration or the like) applied to its root portion. The resulting structure can be obtained. Further, a connecting portion 279 having a cross-sectional shape recessed inside is also connected between the front end surface 276 of the flange portion 274 (the surface facing the protruding front end side of the ignition portion) and the side surface 285 of the base portion 284. If provided, the structure in which the ignition part 280 can withstand the load applied near the boundary between the pedestal part 284 and the collar part 274 is desirable.

As described above, an evaluation test was conducted to confirm the effect of providing the provision in the form of the melting portion 83 formed in the ignition portion 80 provided in the ground electrode 30 of the spark plug 100. First, the relationship between the inclination of the exposed surface 88 of the melting portion 83 (depending on the external angle θ) and the peel resistance, and the linear expansion of the forming material of the discharge portion 81 and the forming material of the pedestal portion 84 constituting the ignition portion 80. The relationship between the coefficient difference and the peel resistance was evaluated. In this evaluation test, materials consisting of four types of noble metal alloys having linear expansion coefficients at 1000 ° C. of 8.3, 9.7, 10.4, 13.4 (× 10 ⁻⁶ ) [1 / K] are prepared. And the discharge part which made the outer diameter S 0.7mm from each material was produced. Moreover, the ground electrode was produced using the Ni alloy whose linear expansion coefficient in 1000 degreeC is 17.8 * 10 < ^-6 > [1 / K], The press part was given to the inner surface, and the base part was formed. Then, the discharge part is arranged on the pedestal part, laser light is irradiated from the side surfaces of the both sides toward the mating surface, laser welding is performed over the entire circumference, the two are joined, and the ground electrode having the ignition part formed on the inner surface The evaluation material (sample) was prepared. Note that the melted portion formed between the pedestal portion and the discharge portion has a formation depth (length T extending radially inward) satisfying S / T = 1 (that is, in the cross section of the ignition portion). The irradiation position, the irradiation angle, the output, the irradiation time, etc. of the laser beam are appropriately adjusted so that the outer angle θ is appropriately changed (with the molten portion on one side and the molten portion on the other side continuous with respect to the central axis P). Adjusted. And about each produced sample, the part where the linear distance between the position X1 and the position X2 becomes the largest was specified, and the external angle (theta) of the angle which the virtual line Q and the central axis P make was measured.

  Next, a heating / cooling test was performed on each sample. Each sample was heated with a burner for 2 minutes so that the final temperature was 1100 ° C. together with the ignition part, and then cooled (slow cooling) for 1 minute in an atmospheric temperature atmosphere. Then, the ignition part of each sample was cut | disconnected in the cross section which passes along the central axis P, and the fusion | melting part was observed using the magnifier. Then, in the melted part, observe the part where cracks, peeling, etc. occurred, classify the occurrence part into the vicinity of the boundary between the discharge part and the melted part, and the vicinity of the boundary between the pedestal part and the melted part. Was measured.

Here, the sample ignition part 380 shown in FIG. 6 will be described as an example. In the cross section including the central axis P of the ignition part 380, on one side in the radial direction with respect to the central axis P (on the left side in FIG. 6), Based on the position X2 of the boundary between the discharge part 381 and the melting part 383 (the position of the boundary between the side surface 382 and the exposed surface 388) X2, the length that the melting part 383 extends radially inward is T1, and the other side (FIG. 6). In the right side), T2. In addition, a length in which a crack or a peeling that occurs on the boundary side between the discharge part 381 and the fusion part 383 extends in the radial direction on one side in the radial direction with the central axis P as a boundary is A1, and on the other side is A2. . And the ratio (oxidation scale) of the length of the crack and peeling which arose in the boundary side of the discharge part 381 and the fusion | melting part 383 is calculated | required with the following formula | equation.
{(A1 + A2) / (T1 + T2)} × 100 [%] (1)

Next, as described above, on the one side in the radial direction with respect to the central axis P, melting is performed on the basis of the position of the boundary between the pedestal portion 384 and the melting portion 383 (the position of the boundary between the side surface 385 and the exposed surface 388) X1. The length of the portion 383 extending radially inward is U1, and the other side is U2. Further, the length in which the cracks and separations that have occurred on the boundary side between the pedestal portion 384 and the melting portion 383 on the one side in the radial direction with the central axis P as the boundary extend in the radial direction is B1, and the other side is B2. . Then, an oxide scale generated on the boundary side between the pedestal portion 384 and the melting portion 383 is obtained by the following equation.
{(B1 + B2) / (U1 + U2)} × 100 [%] (2)

  The ratio of the length of cracks and separation generated on the boundary side between the discharge part 381 and the melting part 383 obtained by the expression (1), and the base part 384 and the melting part 383 obtained by the expression (2) The ratio of the length of cracks and peeling occurring on the boundary side is compared. And the larger one of the ratios of the two kinds of cracks and peeling length is adopted as the oxide scale in the ignition part.

  When the oxidation scale of the ignition part is less than 25%, it is evaluated as “◎” because there is no problem even if cracking or peeling occurs, and when it is 25% or more and less than 50%, it is evaluated as “◯” because the influence is small. . However, when the oxide scale was 50% or more, it was evaluated as “x” because there was a possibility that the discharge part might fall off. The results of this evaluation test are classified according to the magnitude of the outer angle θ of the angle formed by the imaginary line Q and the central axis P, and the difference in linear expansion coefficient between the discharge portion forming material and the base portion forming material. It was shown to.

As shown in Table 1, in all the samples in which the magnitude of the outer angle θ formed by the imaginary line Q and the central axis P is less than 135 °, the oxidation scale of the ignition part is 50% or more. Further, it was found that most of the samples having an outer angle θ exceeding 175 ° had an oxidation scale of 50% or more in the ignition portion, which was not desirable in terms of peeling resistance. On the other hand, it was confirmed that any sample having an outer angle θ of 135 ° or more and 175 ° or less had an oxidation scale of the ignition portion of less than 50%, and a good result in peeling resistance could be obtained. Among samples having an outer angle θ of 135 ° or more and 175 ° or less, samples having a difference in linear expansion coefficient between the discharge portion forming material and the base portion forming material of 8.1 × 10 ⁻⁶ [1 / K] or less The oxidation scale of the ignition part remained below 25%. Therefore, if the difference in linear expansion coefficient between the material for forming the discharge part and the material for forming the pedestal is set to 8.1 × 10 ⁻⁶ [1 / K], it is confirmed that even better results can be obtained in terms of peel resistance. did it.

Next, the relationship between the length (formation depth) at which the melted portion 83 extends radially inward and the peel resistance was evaluated. In this evaluation test, two types of discharges were used, which were made of a Pt alloy having a linear expansion coefficient of 10.4 × 10 ⁻⁶ [1 / K] at 1000 ° C. and having an outer diameter S of 0.7 mm and 1.2 mm. Part was produced. Moreover, the ground electrode was produced using the Ni alloy whose linear expansion coefficient in 1000 degreeC is 17.8 * 10 < ^-6 > [1 / K], The press part was given to the inner surface, and the base part was formed. The discharge part is arranged on this pedestal part, laser light is irradiated from the side surfaces of the both sides toward the mating surface, laser welding is performed over the entire circumference, the two are joined together, and the ground electrode having the ignition part formed on the inner surface An evaluation material (sample) was prepared. At this time, the output (intensity) of the laser beam was appropriately changed so that the formation depth of the melted portion to be formed was appropriately different. Then, in the same manner as in Example 1, the magnitude of the outer angle θ of the angle formed by the virtual line Q and the central axis P was measured, and a sample satisfying 135 ° ≦ θ ≦ 175 ° was extracted as an evaluation target.

  Next, the same heating and cooling test as in Example 1 was performed on each extracted sample. Thereafter, the ignition portion of each sample is cut along a cross section passing through the central axis P, the melted portion is observed using a magnifying glass, and the formation depth of the melted portion (length T extending radially inward) is measured. And the melted portion formation ratio T / S was determined. Furthermore, in the melted part of each sample, observe the part where cracks, peeling, etc. occurred, classify the occurrence location near the boundary between the discharge part and the melted part, near the boundary between the pedestal part and the melted part, The length in the radial direction was measured. And the ratio (oxidation scale) of the crack and peeling length which arose in the ignition part was calculated | required using the above-mentioned formula (1) and (2), and evaluation similar to Example 1 was performed. The results of this evaluation test are shown in Table 2.

  As shown in Table 2, it was confirmed that samples 3, 4, 7, and 8 having a melted portion formation ratio T / S of 0.70 or more had an oxide scale of less than 25%, and good results in peeling resistance could be obtained. did it. In addition, as in Samples 2 and 6, it was found that the oxide scale can be suppressed to less than 50% when the melted portion formation ratio T / S is 0.50 or more. However, as in Samples 1 and 5, it was found that when the melted portion formation ratio T / S is less than 0.50, the oxidized scale of the ignition portion is 50% or more, which is undesirable in terms of peeling resistance.

DESCRIPTION OF SYMBOLS 10 Insulator 12 Shaft hole 20 Center electrode 30 Ground electrode 31 Tip part 33 Inner surface 50 Main metal fittings 80, 180, 280 Ignition part 81 Discharge part 82 Side surface 83 Melting part 84 Base part 85,285 Side surface 86 Projection tip 87 Side surface 89,289 Connecting Portion 100 Spark Plug 274 Hook 276 Tip Surface 279 Connecting Portion

Claims (6)

A central electrode, an axial hole extending along the axial direction, an insulator that holds the central electrode inside the axial hole, a metal shell that surrounds and holds the insulator in the circumferential direction, and one end portion of which A ground electrode joined to the metal shell and bent so that one side surface of the other end portion faces the tip of the center electrode, and on the one side surface of the other end portion of the ground electrode, the center A spark plug provided at a position facing the tip portion of the electrode and projecting from the one side surface toward the central electrode, wherein the ignition portion is directed from the one side surface toward the central electrode. A projecting pedestal, a discharge part joined to the projecting tip of the pedestal by laser welding and forming a spark discharge gap between itself and the tip of the center electrode, the pedestal and the discharge unit Between And a melting part formed by melting both constituent materials by laser welding. The ignition part along a direction in which the ignition part protrudes from the one side surface of the ground electrode. When the arbitrary cross section of the ignition part including the central axis of the ignition part is viewed, the melting part is formed from the side surface of the ignition part toward the central axis, and the arbitrary cross section of the ignition part When the contour shape is viewed, the melting portion has a shape connected to the side surface of the pedestal portion and the side surface of the discharge portion. Further, in the arbitrary cross section of the ignition portion, On one side, the boundary position between the pedestal part and the melting part is X1, the boundary position between the discharge part and the melting part is X2, and the boundary positions X1 and X2 in the arbitrary cross section. Linear distance to When the first cross section that is the largest is viewed, the melted portion is determined based on the outer diameter S of the discharge portion in the radial direction orthogonal to the central axis and the position X2 of the boundary between the discharge portion and the melted portion. A length T extending radially inward satisfies T / S ≧ 0.5, and
A virtual line passing through the positions X1 and X2 of the boundary, external angle theta of the central axis and forms angle, meets the 135 ° ≦ θ ≦ 175 °,
The side surface of the pedestal portion and the one side surface of the ground electrode provided with the pedestal portion are connected by a first connecting portion having a curved shape in which a shape of a cross section including the central axis of the ignition portion is recessed inward. Spark plug characterized by being . A central electrode, an axial hole extending along the axial direction, an insulator that holds the central electrode inside the axial hole, a metal shell that surrounds and holds the insulator in the circumferential direction, and one end portion of which A ground electrode joined to the metal shell and bent so that one side surface of the other end portion faces the tip of the center electrode, and on the one side surface of the other end portion of the ground electrode, the center A spark plug provided at a position facing the tip portion of the electrode and projecting from the one side surface toward the central electrode, wherein the ignition portion is directed from the one side surface toward the central electrode. A projecting pedestal, a discharge part joined to the projecting tip of the pedestal by laser welding and forming a spark discharge gap between itself and the tip of the center electrode, the pedestal and the discharge unit And a melted portion formed by melting the constituent materials of each other by laser welding, and the ignition portion extends in a direction protruding from the one side surface of the ground electrode. When the arbitrary cross section of the ignition part including the central axis of the ignition part is viewed, the melting part is formed in a form from the side surface of the ignition part toward the central axis, and the arbitrary part of the ignition part When the profile shape of the cross section is viewed, the melting portion has a shape connected to the side surface of the pedestal portion and the side surface of the discharge portion, and further, in the arbitrary cross section of the ignition portion, The position of the boundary of the arbitrary cross section while X1 is the position of the boundary between the pedestal part and the melted part and X2 is the position of the boundary between the discharge part and the melted part on one of the side surfaces Between X1 and X2 When the first cross section having the maximum linear distance is viewed, the melting is performed based on the outer diameter S of the discharge portion in the radial direction orthogonal to the central axis and the position X2 of the boundary between the discharge portion and the fusion portion. And the length T extending inward in the radial direction satisfies T / S ≧ 0.5,
  An outer angle θ of an angle formed by an imaginary line passing through the boundary positions X1 and X2 and the central axis satisfies 135 ° ≦ θ ≦ 175 °,
  The pedestal portion has a flange portion whose outer diameter is enlarged on the one side surface side of the ground electrode,
  A surface of the base portion facing the protruding tip side in the flange portion and a side surface of the pedestal portion at the protruding tip end from the flange portion are recessed inward in a cross-sectional shape including the central axis of the ignition portion. A spark plug characterized by being connected by a second connecting portion having an elliptical shape. Among the plurality of cross-sections observed at different circumferential positions around the central axis, the cross-sections of more than half of the entire circumference are the outer diameter S and the length T. DOO together with satisfy T / S ≧ 0.5, spark plug according to claim 1 or 2, wherein the external angle theta is to satisfy the 135 ° ≦ θ ≦ 175 °. The difference between the linear expansion coefficient of the material constituting the discharge part of the ignition part and the linear expansion coefficient of the material constituting the pedestal part is 8.1 × 10
The spark plug according to any one of claims 1 to 3 , wherein the spark plug is ^-6 [1 / K] or less. The discharge part of the ignition part is made of a single noble metal of any one of Pt, Ir, Rh or Ru, or made of a noble metal alloy containing at least one of the noble metals. The spark plug according to any one of claims 1 to 4 , wherein: The pedestal portion has a flange portion whose outer diameter is enlarged on the one side surface side of the ground electrode,
  A surface of the base portion facing the protruding tip side in the flange portion and a side surface of the pedestal portion at the protruding tip end from the flange portion are recessed inward in a cross-sectional shape including the central axis of the ignition portion. The spark plug according to claim 1, wherein the spark plugs are connected by a second connecting portion having a curved shape.

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